Optical communications systems typically comprise of a transmitter which converts electrical signals into optical signals, an optical link over which the optical signals are transported and a receiver which converts the optical signal into an electrical signal, the electrical signal recovered at the receiver ideally being identical to the electrical signal originating at the transmitter. The optical link generally consists of optical transmission fibre to convey the signal and optical amplifiers to compensate for the loss that the fibre introduces. The optical amplifiers are generally interspersed at regular intervals along the link (e.g. every 40-200 km).
One impairment the optical fibre may exhibit is chromatic dispersion. This is where different optical frequencies of the light propagate at different speeds in the fibre. This results in the optical signal being distorted. Eventually after propagating over some distance of fibre, the signal will not be recoverable unless some compensation of this effect is afforded. For example, if a data rate of 10 GBit/s is used in an intensity modulation format at 1550 nm in standard fibre, then the signal will be irrecoverable after around 200 km without some form of compensation. Commercially deployed telecommunications systems use optical dispersion compensation devices to overcome this limitation. There are a number of optical dispersion compensation devices which fulfil this role, the preferred type being dispersion compensating fibre modules. These modules consist of optical fibre of a different type to the transmission fibre, known as dispersion compensating fibre. This fibre has dispersion which is of the opposite sign to the transmission fibre, such that when the dispersion compensating fibre is coupled to the transmission fibre, the overall net dispersion is within the limit of an uncompensated system. Typically the fibre has a higher magnitude of dispersion to the transmission fibre, to reduce the length required. The fibre is generally coiled and placed in modules. A number of modules may be used, distributed at the amplifier sites and terminal sites, or even lumped at one or a few sites.
However, there are a number of drawbacks to using dispersion compensating fibre modules for compensating for the optical dispersion. The most obvious disadvantage is that they tend to be very costly. However, they are also relatively large and have high optical loss. This latter feature results in more optical amplification being required which in turn increases cost and can have degrading effect on performance. A further disadvantage is that dispersion compensating fibre modules tend to be more sensitive to nonlinear distortion, thus also reducing performance. It is therefore desirable to reduce in number or eliminate completely the dispersion compensating devices to reduce the system cost and simultaneously increase the system performance.
One method for removing the optical dispersion compensation devices is disclosed in U.S. patent application Ser. No. 10/262,944, filed on Oct. 3, 2002 and published as US 2004/0067064 A1 on Apr. 8 2004. In this application, instead of compensating for the optical dispersion using an optical dispersion compensating device, the dispersion is compensated by electrical means. Such a method is illustrated in FIG. 1. In this example the electrical signal applied to the optical transmitter is first pre-distorted by applying a digital filter. The pre-distortion function is such that after the signal has passed through the system, it can be recovered at the optical receiver. One feature of this prior art is that to adequately compensate for the dispersion of the system it is necessary to have a pre-distorted signal which modulates both the amplitude and phase of the optical signal. That is the signal applied to the modulator is a complex signal, such that it contains both amplitude and phase information in a polar coordinate system or In-phase and Quadrature components in a Cartesian coordinate system. Accordingly, it is necessary to use an optical modulator which takes a complex signal as its input, a complex modulator. Such a modulator is considerably more expensive to fabricate than a conventional modulator, as the structure is larger and there are more processing steps involved in its fabrication. Additionally a significant expense is the amplifier used to provide sufficient voltage swing to drive the amplifier. Since two such amplifiers are required, this expense is increased. A disadvantage of using a complex modulator is that its optical loss is generally significantly higher than a conventional one. A further disadvantage is that the fabrication steps involved are very different from that of a semiconductor laser, such that it has not been possible to monolithically integrate a complex modulator with a semiconductor laser on the same substrate.